Voltage monitoring circuits are used, for example in systems in which switching voltages are supplied to switchable loads. In such circuits the supplied switching voltages may not exceed a certain adjustable maximum limit value. Therefore these switching voltages must be monitored. If the monitored voltage exceeds the limit value, the monitoring circuit detects the situation and thus enables taking suitable steps for protecting, for example individual circuit components against damage or destruction.
Such monitoring circuits may also be used for a closed loop control of a voltage. The actual value of the voltage is ascertained by the monitoring circuit which provides a closed loop control signal for the voltage to be regulated.
The switching voltage to be monitored is customarily evaluated by an evaluating unit which may be realized as an integrated circuit. Generally inputs of integrated circuits are sensitive relative to excess voltages. The permissible input voltage range is normally limited upwardly by the power supply voltage of the circuit. In case the switching voltage to be monitored becomes higher than the power supply voltage of the integrated evaluating circuit, it may become necessary to reduce the switching voltage being monitored or to provide a voltage transformation. The voltage range in which an evaluation of the switching voltage is reasonably possible, lies normally approximately in the middle between a reference voltage and the power supply voltage of the evaluation unit or circuit. For example, if the power supply voltage is 5 V, the range of the switching voltage to be monitored is between 2 and 3 V.
A voltage divider may be used for reducing or transforming the voltage to be monitored. The voltage divider produces from the switching voltage that needs to be monitored a derived or reduced voltage at the tap of the voltage divider. This derived voltage is supplied to an evaluating unit where the derived voltage that represents the switching voltage to be monitored is, for example, compared with a limit voltage value or with a reference voltage value. In case the switching voltage to be evaluated exceeds the reference value suitable measures may be taken to prevent damage, for example by an emergency circuit switch-off.
However, a voltage divider used conventionally generates a continuous current which may become a critical value due to a continuous power dissipation that may have an adverse effect when used particularly in battery operated systems which have a so-called standby operating feature in which the system may have to remain for prolonged periods of time in order to be ready for normal operation. During such prolonged periods of time power dissipation must be minimized.